LEDs (light-emitting diodes) are semiconductor devices that function as diodes but with the primary feature of emitting light when an electric current passes through them.
When working with high-power LEDs, take appropriate safety precautions to protect your eyes. High-intensity LED light can cause eye damage, similar to the effects of laser exposure. Avoid direct eye contact with the LED beam, and do not look at the light source without proper eye protection. Use appropriate shielding or diffusers to minimize exposure, especially when working at close range or with multiple LEDs. Always follow manufacturer guidelines and ensure a safe working environment to prevent potential harm.
Overview
The first practical LED was invented in 1962 by Nick Holonyak Jr. while working at General Electric.
LEDs produce light when electrons recombine with holes in the semiconductor material, releasing energy as photons—a process called electroluminescence. The color of the emitted light depends on the semiconductor material’s bandgap.
Their conversion of electrical energy into light is highly efficient, meaning that LEDs typically generate minimal heat (though high-power LEDs can still produce significant heat). This characteristic is why LEDs are often described as producing cold light.
Laser Diodes
Laser diodes are closely related to LEDs as both are semiconductor devices that emit light when current passes through them. However, laser diodes differ from LEDs in that they produce coherent, highly collimated light through stimulated emission, whereas LEDs produce incoherent, diffused light through spontaneous emission.
Laser diodes work on a similar fundamental principle as LEDs—electroluminescence in a semiconductor—but with a key difference: stimulated emission. While LEDs emit incoherent light in all directions, laser diodes have a built-in optical cavity that amplifies light through stimulated emission, producing coherent, monochromatic, and highly directional light. Like LEDs, laser diodes have a p-n junction and emit photons when electrons recombine with holes, but their design enables optical feedback and gain, allowing the light to build up in intensity before escaping as a laser beam. The emitted wavelength depends on the semiconductor material, just like in LEDs.
Normal Diodes
LEDs and Laser Diodes belong to the family of diodes, meaning they conduct current only in one direction. However, since LEDs are optimized for light emission rather than blocking reverse voltage, their reverse voltage rating is typically very low, often around 5-7V. Unlike standard diodes, which are designed to withstand reverse voltage safely, LEDs are much more vulnerable and can degrade or fail quickly if exposed to excessive reverse voltage.
This means an LED can only block reverse voltage up to 5-7V before it starts conducting in reverse, potentially overheating and becoming damaged. In AC applications, where voltage polarity constantly alternates, this repeated stress can quickly destroy the LED.
For this reason, LEDs should never be connected directly to 110V/220V AC through a simple resistor. This would not only expose them to excessive reverse voltage but also result in severe inefficiencies. Using a 33kΩ resistor to power an LED from 110/220V AC works but would cause noticeable flickering, waste significant energy as heat, and likely cause the resistor to overheat. Instead, for AC-powered LEDs, proper circuit designs include anti-parallel protection diodes, bridge rectifiers, or capacitive droppers to ensure stable and efficient operation.
More About Forward Voltage, Reverse Voltage, and Breakdown Voltage
Every material has a breakdown voltage, the point at which it becomes conductive.
Semiconductors, including LEDs, have two breakdown voltages:
- The forward voltage is the breakdown voltage for normal operation, allowing current to flow from + to - (the intended direction).
- The reverse voltage is the breakdown voltage when current attempts to flow in the opposite direction.
When an LED is connected correctly, it begins to emit light once the applied voltage exceeds its forward voltage. Typical forward voltages range from 1.6V to 4.0V, depending on the color and semiconductor materials used.
If an LED is connected backward (reverse polarity), it acts as a diode and initially does not conduct electricity. However, unlike purpose-built diodes, LEDs have a very low reverse voltage threshold—typically around 5V.
If the reverse voltage exceeds this limit, the LED begins to conduct, but instead of emitting light, it dissipates power as heat, ultimately destroying the LED.
Using a current-limiting series resistor or another current-limiting method can help protect against accidental reverse voltage. Even if the voltage exceeds the reverse threshold, only a small current will flow—insufficient to cause significant damage.
LED Technologies
LED technology has advanced significantly over the years. Today, LEDs are affordable and available in various shapes and forms, ranging from simple indicator LEDs to advanced applications such as COBs (Chip-on-Board LEDs), LEPs (Laser-Excited Phosphor), programmable LED strips, and OLED displays.
| Technology | Year of invention | Description |
|---|---|---|
| LED | 1962 | The invention of semiconductors that emit light. Initially limited to a few colors, later advancements enabled LEDs to emit nearly any visible color. |
| Laser Diodes | 1962 | Semiconductor devices that produce coherent, highly collimated light through stimulated emission. Used in optical communication, barcode scanners, and laser pointers. |
| Seven-Segment Displays | 1969 | LEDs integrated into display modules and driven by a multiplexer to show simple numbers and characters. |
| Dot-Matrix Displays | 1970s | Arrays of LEDs arranged in an 8×8 (or similar) grid, driven by a multiplexer, capable of displaying simple graphics and symbols. |
| COB (Chip-on-Board) | 1980s | High-density LED packaging where multiple LED chips are mounted directly onto a substrate, creating a seamless light-emitting surface. |
| Programmable LEDs | 2000s | LEDs with built-in driver circuits, often in RGB, RGBW, or RGBWW configurations (e.g., WS2812), using single- or two-wire protocols. These allow large numbers of LEDs to be controlled via a single GPIO, commonly found in LED strips and matrices. |
| OLED Displays | 1987 | High-resolution displays composed of self-illuminating organic LEDs, eliminating the need for a backlight. |
| LEP (Laser-Excited Phosphor) | 2000s | High-intensity, highly focusable light sources using a blue laser diode to excite a phosphor substrate, producing white or colored light. |
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The HP Model 5082-7000 Numeric Indicator, introduced in 1969, was among the first LED devices to use integrated circuit technology, paving the way for seven-segment displays.
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Organic Light-Emitting Diode (OLED) technology was first demonstrated in 1987 by Ching W. Tang and Steven Van Slyke at Eastman Kodak. It took until the 2000s until OLED technology was affordable and robust enough for widespread adoption.
Other display types, such as LCD and TFT screens, rely on liquid crystal technology, which differs from LED-based displays. These displays do not emit light themselves but instead use a backlight, with liquid crystals controlling which areas allow light to pass through.
Rugged Yet Easy to Destroy
LEDs are rugged, long-lasting, and more resilient to mechanical stress than most other light sources. However, despite their durability, they can be easily destroyed if not used correctly.
Low Internal Resistance
One key characteristic makes LEDs delicate and sensitive: they have very low internal resistance.
If an LED is connected directly to a power source, it will almost instantly burn out. Due to its low resistance, an LED behaves similarly to a wire or a fuse—when exposed to excessive current, it heats up rapidly and is permanently damaged.
Always Control Current
To operate an LED safely, you must always limit the current flowing through it. There are various methods to achieve this, but in hobbyist projects, a simple series resistor is the most common solution.
Identifying Anode and Cathode
Since LEDs are semiconductors that conduct current in only one direction, it is crucial to connect them with the correct polarity. The two legs of an LED are called the anode (+) and the cathode (-).
Standard Indicator LEDs
Indicator LED are cheap and simple to use. Many hobbyist projects use these.
Straw-Hat LED
Straw Hat LED have a shape resembling a straw hat, partially because they feature a built-in lens for a wide viewing angle.
Piranha / SuperFlux LED
Piranha LED are small and compact square LED with a relatively large light output with four pins (instead of two). They are also known as SuperFlux.
Dual Color LED
Dual color LED are really just two-in-one LED that internally consist of two regular LED in different colors. They are perfect for indicating two different states, i.e. funtional (green) and error (red).
RGB Color LED
RGB LED can produce any color: three internal LED in red, green and blue mix any other color.
Programmable LED
Once LED consist of more than one color, they become difficult to operate: each internal color LED has its own specific forward voltage and needs its own specific voltage.
Worse, LED strip multiply this effort: each of the three color LED in each of the connected RGB LED need to be carefully wired.
Programmable RGB LED come to the rescue by adding a tiny chip to each programmable RGB LED. Both color mixing and current control is managed by this chip. It is controlled by just one data pin that can be daisy chained to string up any number of programmable RGB LED.
Most commonly, the LED controllers are embedded in SMD LED like the type 5050 in the picture above.
Programmable LED are comprised of a LED and a controller chip. From the outside, you can only see the LED. This is why regular SMD LED are indistinguishable on first sight from programmable SMD LED.
The LED controller chip can essentially be embedded in any LED and are not restricted to LED strips. They are i.e. also available as single discrete LED*.
SMD LED
SMD (surface mounted device) LED are a special form factor and suitable for direct mounting (soldering) to PCB. This SMD form factor is available for any of the discussed LED types:
You get SMD versions of single color, RGB, and programmable LED.
3W High Performance LED (Generic)
LED can be used for illumination, too. These high performance LED turn much higher currents into light than the typical 10-20mA used by simple indicator LED.
COB LED
With dual color and RGB LED, you have seen multiple individual LED blended together. COB (Chip On Board) takes this a step further and mounts a large number of LED directly onto a substrate or circuit board.
The result are large surfaces of any shape or area, a cheap production method and very much homogenous light output.
The many different LED on a COB are partially connected in series and partially connected in parallel to design a particular desired total forward voltage. They often can be connected directly to voltages like 12-13V without the need of series resistors or constant voltage/constant current power supplies.
7-Segment Displays
Multiple LED are combined in 7-Segment displays. They are commonly used to display numeric information.
Dedicated controller IC make it simple to drive these displays via I2C or similar interfaces.
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(content created Mar 17, 2024 - last updated Mar 05, 2025)